This invention relates to limited-rotation motors or galvanometers, and more particularly to that class in which the rotor is supported on ball bearings. The invention also relates to a novel bearing unit that uses a preload characterized by a high spring constant.
These devices often perform precision tasks that require a high degree of accuracy. Common uses of these devices include electronic manufacturing and repair operations in which a laser beam is directed to perform tasks such as the profiling, marking, cutting, drilling, and trimming of silicon and other semi-conducting materials. The galvanometers are also used in high-precision optical scanning operations in which a laser beam is swept over a field of interest and reflections of the beam are sensed and analyzed. To perform these precision tasks, limited-rotation motors are equipped with means for monitoring and reporting the instantaneous angular positions of the rotors.
As described herein, the rotor may comprise a cylindrical permanent magnet armature and a pair of shafts that support the armature for rotation in ball bearings. Several factors influence the service life of these bearings and the axial position of the rotor, which in turn affect the accuracy of these devices. For example, in high precision applications of the galvanometer, bearing slackness, which results in radial movements of the rotor, results in substantial degradation of the accuracy of the beam positioning. Therefore, the bearings are preloaded by means of a spring arrangement to remove the slack. However, conventional preloading systems do not perform satisfactorily in the applications to which this invention is directed.
More specifically, a substantial amount of heat is generated in a galvanometer in a continuous-use environment such as high-speed scanning. The resulting temperature changes cause expansion and contraction of the armature. If the preload spring has a high spring constant, these dimensional changes can cause a relatively large change in the force exerted by the preload spring, resulting in excessive bearing wear in one direction and inadequate preload in the other direction.
A preload spring having a relatively low spring constant will avoid excessive changes in the preload force in response to thermal expansion and contraction. However, the low spring constant causes another problem. Because of various asymmetries in the system, a time-varying axial force is exerted on the rotor. The resulting axial displacement of the rotor is a chatter characterized by a resonance involving the mass of the rotor (and load) and the spring constant of the preload spring. If the resonant frequency is within the passband of the servo system that drives the galvanometer, the relatively large axial displacement at this frequency will be sensed by the angular-position sensor that provides a feedback signal. This will cause instability in the servo system. Accordingly, the passband is limited to frequencies below the resonance, which has a low frequency owing to the low spring constant.
In addition, conventional galvanometer design has not recognized that axial placement of the bearings, stator drive coils, and stator back iron can also profoundly affect the axial forces applied to the bearings during acceleration of the rotor assembly. As a result, insufficient attention has been devoted to placing these elements in an axially symmetric relationship with respect to the magnetic center of the armature, and unnecessarily large axial forces have been applied to the bearings, shortening their life considerably.
Therefore, what is needed, is an efficient, high-performance galvanometer bearing assembly design, which extends the service life of the bearings, while providing accurate performance. Such a bearing design would provide the stiffness of a high preload design, thereby avoiding problems resulting from chatter and would also provide the compliance of a low preload design that would account for bearing wear and differential thermal expansion of the rotor components.
A galvanometer as it is to be understood in the present context is a limited-rotation motor equipped with means for monitoring and reporting the instantaneous angular position of the rotor. In the preferred embodiment of the invention, the rotor is equipped with an output shaft, supported by a novel duplex pair bearing unit, to which may be attached an arbitrary load, and a tail shaft supported by a second bearing assembly. The duplex pair bearing assembly has pairs of inner and outer races, wherein a preload is established by urging the pair of inner races, either toward or away from each other, into an offset position from the pair of outer races so as to take up the slack. The inner and outer races are axially restrained with regard to the output shaft and the motor housing, respectively.
The preloaded bearing assembly has a high spring constant, which maintains the rotor in an essentially fixed axial position with respect to the motor housing. Therefore, such axial movement of the rotor that does occur is characterized by a relatively high resonant frequency. In addition, each of the components of the preloaded duplex pair bearing assembly is preferably fabricated from the same material so that the bearing assembly does not incur a change in preload as a result of thermal expansion or contraction.
At the other end of armature, the tail shaft is mounted in a conventional second bearing unit preloaded with a low spring constant relative to that of the first bearing unit. Because of the stiffness the first bearing unit, thermal expansion and contraction of the rotor assembly is taken up almost entirely in the second bearing unit. Because the second bearing unit is preloaded with a spring having a relatively low spring constant, thermal expansion does not cause a large change in the preloading in this unit.
Another important consideration in designing a galvanometer is the proper positioning of the internal components such as the bearings, stator drive coils and stator back iron. Another novel feature of the present invention is that these elements are arranged symmetrically with the magnetic center of the rotor, thereby greatly reducing the parasitic axial bearing forces, and prolonging the life of the bearings.
A further novel feature of the present invention is that the rotor magnet is manufactured to or selected for a high degree of property uniformity along its length. As the property uniformity of the magnet increases so does the torque constant of the rotor. An increase in magnetic property uniformity also decreases the inherent axial loading of the bearings, thereby facilitating the design and use of the preloading spring for the second bearing unit.